1. Field of the Invention
The present invention relates to a process for the preparation of an aromatic polyimide film having a surface provided with extremely small sized protrusions, and more particularly to a process for the preparation of an aromatic polyimide film which is useful as a base film of a recording medium employed in high-density recording system.
2. Description of Prior Art
A polyethylene terephthalate film has been heretofore used as a support (i.e., base film) of a magnetic recording medium, because the polyethylene terephthalate film is excellent in various properties such as heat resistance, mechanical strength and surface smoothness, and further it is obtainable at a low cost. Recently, there is demand for a higher density recording system, prolonged recording period, higher quality of recorded image and a smaller sized and light-weight medium, and hence a magnetic recording medium having a metal thin layer of a magnetic recording material such as Co, Cr, Ni, Fe or an alloy thereof formed on a base film by sputtering or vacuum deposition has been widely studied.
In the case of forming the metal thin layer (i.e., magnetic recording layer) on the base film by the process such as sputtering, the temperature of the base film rises to over 200.degree. C. due to an energy given by collision of metal particles to the film, even when the base film is not heated. Further, depending on the nature of the employed metal, the base film may be heated at a temperature of higher than 200.degree. C. to form a metal thin layer thereon in order to improve magnetic properties such as coercive force or to improve adhesion between the metal thin layer and the base film. Accordingly, as the base film of a magnetic recording medium in which a metal thin layer (magnetic recording layer) is formed by the process such as sputtering or vacuum deposition, the polyethylene terephthalate film is not satisfactory because the film does not have sufficient heat resistance. There is known a process where a magnetic recording layer is formed on a base film having poor heat resistance such as a polyethylene terephthalate film by sputtering and the like, under the condition that the base film is cooled during the sputtering and the like. However, the magnetic recording layer obtained in this process sometimes decreases in the magnetic properties or shows unsatisfactory adhesion to the base film, so that this process is unsuitable as a process for preparing a magnteic recording medium of high qualities.
For these reasons, it has been proposed to use an aromatic polyimide film having high heat resistance as a base film of a magnetic recording medium of high qualities.
It is generally known that various properties of magnetic recording media such as electromagnetic conversion characteristics, smooth running property and running endurance greatly depend on the surface condition of the magnetic recording layer, and hence many studies for improvement of the surface condition of the magnetic recording layer have been made.
A metallic recording layer of a magnetic recording medium generally has a thickness of approx. 1,000 to 5,000 angstroms, and the layer is prominently thinner than that of the coated magnetic recording layer of a conventional recording medium. Hence, the surface conditions of the base film are reproduced on the surface of the magnetic recording layer. Accordingly, the improvement of various properties of the magnetic recording layer (i.e., metal thin layer) such as electromagnetic conversion characteristics, smooth running property and running endurance can be accomplised by providing a favorable shape or conditions to a surface of the base film. In more detail, if the surface of the base film is too rough, that is, the surface of the surface of the recording layer is too rough, the distance between the recording layer and a magnetic head in the recording and reproducing procedures becomes to large, whereby the output level lowers owing to so-called spacing loss. Further, existence of coarse protrusions on the surface of the base film causes drop-out phenomenon. On the contrary, if the surface of the base film is too smooth, there arise other problems such as problems in head-touch property, smooth running property, etc.
For coping with the above-described problems relating to running properties, there has been heretofore proposed a process of incorporating organic or inorganic fine particles into a base film of polyethylene terephthalate to form a number of extremely small sized protrusions on the surface of the base film, or a process of coating a solution comprising organic or inorganic fine particles, a resin binder and a solvent onto the surface of the base film and drying the coated layer of the solution to form a number of small sized protrusions. Particularly, the latter process is very effective for improving the running properties of the resulting magnetic recording medium.
As described hereinbefore, a heat-resistant base film is required to be used for forming a magnetic recording layer thereon by sputtering and the like. However, in the case of using an aromatic polyimide film as the heat-resistant base film, the aforementioned processes for the polyethylene terephthalate film can hardly prepare a desired base film of high qualities.
In more detail, in the former process (process of introducing fine particles into the base film), the strength of the base film is lowered, and hence the resulting magnetic recording medium can be hardly used practically. In the latter process (process of coating a resin binder solution on the base film), the resin binder used therefor is also required to be heat-resistant. However, there is not known a solvent capable of dissolving such heat-resistant resin binder and having wettability to the aromatic polyimide film, so that it is almost impossible to uniformly form a number of extremely small sized protrusions on the surface of the aromatic polyimide film by the latter process.